The present invention relates to a photothermographic material, in particular, a photothermographic material that realizes higher sensitivity. More precisely, the present invention relates to a photothermographic material useful for use in image setters suitable for photomechanical processes, medical diagnosis and so forth.
In recent years, reduction of amount of waste processing solutions is strongly desired in the fields of films for medical diagnosis, photomechanical processes and so forth from the standpoints of environmental protection and space savings. Therefore, photothermographic materials are noted as films for medical diagnosis and photomechanical processes that can be efficiently exposed by using a laser image setter or laser imager and can form clear black images with high resolution and sharpness. Such photothermographic materials can provide a simpler and non-polluting heat development processing system that does not require use of solution-type processing chemicals. Photothermographic materials contain a silver salt of an organic acid, photosensitive silver halide grains, reducing agent and binder on a support, and described in, for example, U.S. Pat. Nos. 3,152,904, 3,457,075 and D. Klosterboer, Imaging Processes and Materials, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d, 8th ed., Chapter 9, page 279, compiled by J. Sturge, V. Walworth and A. Shepp, Neblette (1989).
However, since the photosensitive silver halide contained in photothermographic materials is not fixed and remains in films even after image formation, grain size and amount thereof are limited in order to prevent degradation of printed out conditions. That is, the grain size and amount of photosensitive silver halide are designed so as to be as small as possible. Therefore, photothermographic materials have a problem of lower sensitivity compared with photosensitive materials for wet processing.
For use in photomechanical processes for printing, a substantially colorless photosensitive material (in particular, colorless for the UV region) that can provide high contrast photographic characteristic is required. As for methods of obtaining high contrast photographic characteristic, European Patent Publication EP762,196A, Japanese Patent Laid-open Publication (Kokai, henceforth referred to as JP-A) No. 9-90550 and so forth disclose that high-contrast photographic characteristic can be obtained by incorporating Group VII or VIII metal ions or metal complex ions thereof into photosensitive silver halide grains for use in photothermographic materials, or incorporating a hydrazine derivative into the photothermographic materials. Further, as for a photosensitive material for which exposure with an infrared ray is intended, techniques concerning infrared sensitive photothermographic silver halide photographic materials have been developed, which can markedly reduce absorption in the visible region of sensitizing dyes and antihalation dyes and hence enable easy production of a substantially colorless photosensitive material. Spectral sensitization techniques are disclosed in Japanese Patent Publication (Kokoku, hereinafter referred to as JP-B) No. 3-10391, JP-B-6-52387, JP-A-5-341432, JP-A-6-194781, JP-A-6-301141 and so forth, and antihalation techniques are disclosed in JP-A-7-13295, U.S. Pat. No. 5,380,635 and so forth.
Dyes providing spectral sensitization by infrared absorption generally show high HOMO and hence strong reducing ability, and thus they are likely to reduce silver ions in photosensitive materials to degrade fog of the photosensitive materials. In particular, during storage under high temperature and high humidity or storage for a long period of time, marked change of performance may be observed. Moreover, if a dye showing low HOMO is used in order to prevent the degradation of storability, there is caused a problem that LUMO also correspondingly becomes lower, spectral sensitization efficiency is reduced and hence sensitivity is lowered.
In the fields of newspaper printing and facsimile utilizing photomechanical processes, higher processing speed is preferred for photomechanical processing systems, and therefore a technique of providing a photothermographic material of high sensitivity has been desired. Considering these problems of the prior art, an object of the present invention is to provide a photothermographic material of high sensitivity. Another object of the present invention is to provide a photothermographic material useful for medical use, which exhibits high sensitivity and provides gradation suitable for diagnosis.
As a result of assiduous studies of the inventors of the present invention, it was found that high sensitivity could be realized by a photothermographic material containing a particular compound, and they accomplished the present invention.
That is, the present invention provides a photothermographic material containing a silver salt of an organic acid, a photosensitive silver halide, a reducing agent and a binder on a support, which contains at least one compound selected from compounds of the following Types (i) to (iv).
Type (i)
A compound of which one-electron oxidized derivative produced by one electron oxidation of the compound is capable of releasing two or more electrons with a bond cleavage.
Type (ii)
A compound of which one-electron oxidized derivative produced by one electron oxidation of the compound is capable of releasing one more electron with a bond cleavage and which has two or more groups adsorptive to silver halide in the molecule.
Type (iii)
A compound of which one-electron oxidized derivative produced by one electron oxidation of the compound is capable of releasing one or more electrons after undergoing a bond formation process.
Type (iv)
A compound of which one-electron oxidized derivative produced by one electron oxidation of the compound is capable of releasing one or more electrons after undergoing an intramolecular ring cleavage reaction.
In the present invention, the compounds of Types (i) to (iv) are preferably compounds represented by the following formulas (1-1) to (4-2). 
In the formula (1-1), RED11 represents a reducing group that can be one electron-oxidized, and L11 represents a leaving group. R112 represents a hydrogen atom or a substituent. R111 represents a nonmetallic group that can form a tetrahydro, hexahydro or octahydro derivative of a 5- or 6-membered aromatic ring (including an aromatic heterocyclic ring) together with the carbon atom to which R111 bonds and RED11. 
In the formula (1-2), RED12 represents a reducing group that can be one electron-oxidized, and L12 represents a leaving group. R121 and R122 each independently represent a hydrogen atom or a substituent. ED12 represents an electron donor group. In the formula (1-2), R121 and RED12, R121 and R122 or ED12 and RED12 may bond to each other to form a ring structure. 
In the formula (1-3), Z1 represents an atomic group that can form a 6-membered ring together with the nitrogen atom to which Z1 bonds and two of carbon atoms of the benzene ring, R1, R2 and RN1 each independently represent a hydrogen atom or a substituent, X1 represents a substituent that can substitute on the benzene ring, m1 represents an integer of 0-3, and L1 represents a leaving group. A compound of the formula (1-3) can, after it is one electron-oxidized, further release two or more electrons due to spontaneous cleavage of the C (carbon atom)-L1 bond. 
In the formula (1-4), ED21 represents an electron donor group, R11, R12, RN21, R13 and R14 each independently represents a hydrogen atom or a substituent, X21 represents a substituent that can substitute on the benzene ring, m21 represents an integer of 0-3, and L21 represents a leaving group. RN21, R13, R14, X21 and ED21 may bond to each other to form a ring structure. A compound of the formula (1-4) can, after it is one electron-oxidized, further release two or more electrons due to spontaneous cleavage of the C (carbon atom)-L21 bond. 
In the formula (1-5), R32, R33, R31, RN31, Ra and Rb each independently represents a hydrogen atom or a substituent, and L31 represents a leaving group. However, when RN31 represents a group other than an aryl group, Ra and Rb bond to each other to form an aromatic ring. A compound of the formula (1-5) can, after it is one electron-oxidized, further release two or more electrons due to spontaneous cleavage of the C (carbon atom)-L31 bond. 
In the formula (2-1), RED2 represents a reducing group that can be one electron-oxidized, and L2 represents a leaving group. When L2 represents a silyl group, the compound has two or more of nitrogen-containing heterocyclic groups substituted with a mercapto group as absorptive groups. R21 and R22 each independently represent a hydrogen atom or a substituent. RED2 and R21 may bond to each other to form a ring structure.
A compound of the formula (2-1) is a compound that can, after the reducing group represented by RED2 is one electron-oxidized, further release one more electron due to spontaneous cleavage of the C (carbon atom)-L2 bond. 
In the formula (3-1), RED3 represents a reducing group that can be one electron-oxidized, Y3 represents a reactive group moiety that reacts after RED3 is one electron-oxidized, and L3 represents a bridging group bonding RED3 and Y3. 
In the formulas (4-1) and (4-2), RED41 and RED42 each independently represent a reducing group that can be one electron-oxidized, and R40 to R44 and R45 to R49 each independently represent a hydrogen atom or a substituent. In the formula (4-2), Z42 represents xe2x80x94CR420R421xe2x80x94, xe2x80x94NR423xe2x80x94 or xe2x80x94Oxe2x80x94. R420 and R421 each independently represent a hydrogen atom or a substituent, and R423 represents a hydrogen atom, an alkyl group, an aryl group or a heterocyclic group.
When the photothermographic material of the present invention is subjected to light exposure and heat development at 121xc2x0 C. for 24 seconds, it is preferred that 90% of developed silver grains in terms of grain number should be in contact with the silver halide. Further, an inclination of a straight line connecting points corresponding to Dmin+density 0.25 and Dmin+density 2.0 on the characteristic curve of the photothermographic material is preferably within the range of 2.0-5.0, more preferably within the range of 2.5-3.5. Further, the photothermographic material of the present invention preferably contains a high contrast agent.